Anti-lock brake question

[jackw3306]

In a straight line panic stop would the anti-lock brake system keep the tires from leaving skid marks? Specifically, in a 2002 VW bettle.

 

[ivymike]

not usually.

 

[BobM3]

I've seen quite a few evenly interupted skid marks on pavements so I'd say no.

 

[SomptingGuy]

This sounds like the kind of question a lawyer would be asking. What's the background?

- Steve

 

[MiketheEngineer]

I've locked up a few different vehicles to see what happens - no skid marks..... The wheels keep spinning ever so slightly in order to maintain steering control and to avoid spins or skids.

 

[IRstuff]

Skidding, even partially, would seem to be contrary to the functionality of the ABS.

That said, every manufacturer obviously rolls his own algorithm to control the ABS. There may be sensor failures or processor failures on top of everything else.

TTFN

FAQ731-376

 

[SomptingGuy]

Skidding leaves evidence. Proof that you did actually brake.

- Steve

 

[dgallup]

Most ABS systems detect the difference in wheel speed & reduce pressure in the caliper cylinder before full lockup occurs. Early systems were slower to respond & there may have been full lockup briefly. It also depends on the coefficient of friction & wheel inertia. It is possible to skid slightly without leaving visible tire marks depending on the tire & the road surface.

 

[ivymike]

It's probably also possible to leave tire marks without "full lockup."

I've never had an emergency-braking-to-full-stop situation that didn't leave black stripes on the road, even with ABS (and stability control, and roll control in some cases) in several vehicles I've owned recently. The ones for which specific examples come to mind are Honda Civic, Mini Cooper, Nissan 4wd pickup, Ford Explorer, Toyota Scion Xb. In some cases I left more than 20ft of skid marks. I seem to recall having seen some skid marks with the central zig-zag patterns of the tread visible, which would suggest that the tire was rotating while leaving marks.

 

[izzmus]

It's also possible to lock the brakes without leaving skid marks, if the rubber turns to dust instead of liquefying, the evidence will blow away with the first decent wind.

 

[RossABQ]

I was under the impression some ABS systems stop controlling below some low speed (like 3 mph). That may have been just the early systems.

 

[GregLocock]

Even now they tend to have a speed below which the ABS is disabled - more like 5 mph on Bosch systems that I've seen. The reason is that the early systems had problems differentiating between the vehicle actually stopping and wheels locking, and the happy coincidence that locked wheels are not a bad way of knocking the last bit of speed off, especially on difficult surfaces.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[MacGyverS2000]

I imagine the confusion has to do with the reluctance sensor not receiving pulses fast enough when the vehicle is moving slowly...

Dan - Owner

http://www.Hi-TecDesigns.com

 

[izzmus]

With a scantool, you can usually see speeds down to 1mph with properly aligned/gapped sensors.

As the knuckles rust, it pushes the sensor away from the tone ring and the signal starts dropping out at higher speeds, until the point that the ABS starts activating during normal stops.

I'm sure there's a good reason why some manufacturers still mount the sensors radially instead of axially, but I can't think of one.

 

[GregLocock]

Packaging

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[izzmus]

I still don't buy that. They could just as easily put the teeth on the side of the tone ring and have the sensor read the side instead of the circumference. That way when the sensor gets pushed up by rust, the gap does not change.

Perhaps this is only a regional problem.

 

[MikeHalloran]

In a small sample, I haven't noticed an ABS sensor physically displaced by rust. I have seen them fouled with iron filings, e.g. pieces of brake rotor.



Mike Halloran
Pembroke Pines, FL, USA

 

[GregLocock]

You don't buy that because you have never designed the outboard end of a suspension. Typically ABS was grafted onto an existing knuckle, which limits the options.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Thecardoc]

Mike H. wrote "In a small sample, I haven't noticed an ABS sensor physically displaced by rust. I have seen them fouled with iron filings, e.g. pieces of brake rotor."

Hi Mike. This is an extremely common occurrence around here. Often times removal of the sensor, and cleaning the mounting surface and sensor bore through the knuckle restores normal operation.

ABS modules are very sensitive to flaws in any of the sensor signals. If a single pulse from a sensor is delayed, or simply too weak just prior to a vehicle stopping the module interprets that as a lock-up and the system goes into anti-lock mode. Some manufactures have resorted to disabling the system at speeds of 3 to 5 mph, when a false signal drop-out is most likely to occur.

BTW the best way to tell which signal is dropping out isn't to watch the speed signals, the tech should watch the dump valve commands the ABS module issues. The brake being modulated incorrectly, is the one that the sensor is acting up on.

 

[GregLocock]

"Some manufactures have resorted to disabling the system at speeds of 3 to 5 mph, when a false signal drop-out is most likely to occur.
"

Who doesn't? There are good reasons for disabling the ABS at very low speeds that have nothing to do with signal dropout, I would be very nervous about buying a car that attempted ABS at such low speeds.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.